The present invention relates to a method for calculating color correction conditions, a method for determining an exposure amount for printing, a method for processing images, an image processing apparatus, an image forming apparatus, a printing exposure apparatus and a storage medium.
In a conventional method for printing from a color photographic original, mean density of an entire area of a frame of the color photographic original is measured, and whereby exposure amount is controlled in a way that an exposure amount for each color of blue, green and red given to a light-sensitive material corresponding to the full area of the frame is constant, so that a print having an excellent color balance may be produced. However, when a frame of a color photographic original that is of a high chroma, namely, of a greatly deviated distribution of a color of an object, for example, of a color photographic original wherein the most part of the frame is occupied by a specific chromatic color, is exposed to light, the above-mentioned method reveals a problem that an optimum print is hard to be obtained.
Further, many of inputted color digital images are generally those obtained through image-pickup under a dynamic range that is narrower than a brightness range of an object and is narrower than a density range within which a color film is capable of recording. Therefore, in many cases, pixels with high chroma are converted to recordable colors to be put out of order in reproduction of color balance, and they are deviated in different colors due to a difference in inputting apparatuses or in kinds of color films, to become those with poor image quality wherein colors are deviated when they are outputted as they are.
Therefore, color balance is corrected by various methods for outputting. In these conventional methods, however, it has sometimes been impossible to obtain appropriate results because of a difference in inputting apparatuses or in kinds of color films, or in types of images.
In conventional methods, therefore, inputted digital images are displayed on a monitor, then correction is made through adjustment wherein an operator increases or decreases a data value of each color in image data while the operator observing the displayed image, and a hard copy which is a result of the correction is outputted. Then, the hard copy thus outputted is compared with the image displayed on the monitor, and a difference between them is used for a determination of the final correction, and a hard copy which is a result of the determined final correction is outputted. However, repeated correction and outputting take a long time, and a difference in skills of the operator results in a big difference in color quality of the hard copy obtained, which has been a problem.
For this problem, there has been made a proposal wherein each frame of a color photographic original having a plurality of frames is read, and when determining an exposure amount for obtaining a print from the color photographic original having a plurality of frames through exposure for printing based on each primary color information of each pixel in each frame thus obtained, a density difference between two sets of two primary colors and neutral density are obtained, and a value that is specific to the color photographic original is obtained for the exposure control, from the functional relation of the density difference between two sets of two primary colors and neutral density.
In this method, however, when there is a high chroma frame wherein the rate of high chroma pixels is high in frames of a color photographic original, it has been difficult to obtain a print having correct balance from such high chroma frame because of an influence of the high chroma pixel.
Heretofore, therefore, an influence of a high chroma pixel has been reduced to the utmost extent through the method for taking the statistics by reducing an influence of a high chroma pixel utilizing the correlation coefficient of a cumulative density function between primary colors as stated in Japanese Patent Publication Open to Public Inspection No. 66498/1993 (hereinafter referred to as Japanese Patent O.P.I. Publication), for example, or through the method as disclosed in German Patent No. 2912130 wherein each frame is judged whether it is a high chroma frame or not based on prescribed conditions between densities, then, statistics are taken excluding frames judged to be a high chroma frame, then, each pixel of frames judged not to be a high chroma frame is judged whether it is a high chroma pixel or not based on the statistics mentioned above, and statistics other than aforesaid statistics are taken excluding pixels judged to be a high chroma pixel, whereby an influence of the high chroma pixel may be reduced.
However, when statistics are taken excluding high chroma frames, pixels which are not of high chroma in the high chroma frames are eliminated from the statistics because pixel information on an entire frame is removed. Therefore, an amount of statistics can not be secured sufficiently, an appropriate amount of color correction can not be obtained, and therefore an exposure amount for obtaining images corrected properly in terms of color can not sometimes be obtained. On the other hand, when the condition to evaluate a pixel in a frame which is not high chroma to be high chroma is relaxed for securing an amount of statistics, a pixel which must be evaluated to be high chroma is not also evaluated to be a high chroma pixel, resulting in inability to remove an influence of a high chroma pixel sufficiently, thus, there has been a high rate of occurrence of images whose color balance is poor due to an influence of high chroma pixels.
In the method disclosed in German Patent No. 2912130, high chroma frames are first eliminated on a basis of an entire frame though high chroma pixels in a frame that is not of high chroma are eliminated, therefore, pixels which are not of high chroma included in a high chroma frame are also removed from the statistics, thus, an amount of statistics can not be secured sufficiently, resulting in the rate of occurrence of images having poor color balance which is still high. When the number of objective frames is only one, if the frame is evaluated to be high chroma, no statistics are obtained and an appropriate amount of color correction can not be obtained, which has also been a problem.
Even when statistics excluding high chroma pixels are taken by removing high chroma pixels under the prescribed condition based simply on the maximum density and minimum density, it is difficult to establish the condition for eliminating the high chroma pixels, and pixels which are not of high chroma are removed as a high chroma pixel by color fluctuation based on characteristics of a color film and characteristics caused by processing, and on the contrary, pixels which are primarily high chroma are not removed, thus all prints obtained have only been those whose fraction defective is still high.